Method for enhancing the surface of a substrate and catalyst products produced thereby

ABSTRACT

A method of treating the surface of a substrate by thermally spraying large size particles, &gt;10 micrometers, of a composition such as a metal hydroxide, carbonate, or nitrate directly onto the substrate whereby a small size particle coating, &lt;5 microns and more particularly &lt;3, is formed on the substrate, enhancing the surface area and porosity properties of the substrate, and substrates with metal oxide surfaces produced thereby.

FIELD OF THE INVENTION

[0001] The invention relates to a method for enhancing the surface of asubstrate and products produced thereby. More particularly the inventionrelates to a method of treating a substrate by thermally spraying acatalyst precursor material directly onto the substrate surface toincrease porosity and surface area, and substrates formed thereby. Theinvention is particularly useful in treating a metal separator used in amicro-component reaction chamber to form a catalyst surface coatingthereon.

BACKGROUND OF THE INVENTION

[0002] In the prior art, material coatings typically have been appliedto metallic or other surfaces for protective or restorative purposes,such as wear and corrosion resistance, thermal barrier coatings(“TBCs”), protection against oxidation, and dimensional restoration.Coatings made from ceramic materials in particular offer resistanceagainst abrasive wear and thermal shock and provide electricalinsulation.

[0003] The thermal spray coating process has evolved from a method forpatching or repairing material surfaces to a technique enabling surfacemicrostructure design and enhancement. As is known to those skilled inthe art, thermal spraying involves any of several methods by which acoating material is placed in the path of a spray jet, heated until thematerial softens or melts, and propelled to the surface of a preparedsubstrate to form a deposit. U.S. Pat. No. 5,900,283 to Vakil et al. andU.S. Pat. No. 5,985,368 to Sangeeta et al. describe various methods ofdepositing a protective coating on a metal-based substrate, includingthermal spraying.

[0004] Thermal spraying processes include flame spraying and plasmaspraying. In each process, the material is impacted as it is depositedon the substrate and is allowed to cool and solidify to form a coatingon the substrate. The deformation of the softened or molten materialupon impact with the substrate, and, in some cases, the force of theimpact, is sufficient for the material to mechanically bond to thesurface. In thermal spraying processes, the particles sprayed, uponimpact with the substrate surface mechanically lock and/or bond into theprofile of the surface.

[0005] Initially, thermal spray technology focused on techniques ofheating particles into a plastic state before impact to effect bondingwith the surface. Methods of high velocity spraying now exist wherebyextreme spray velocities are generated. Powder material is injected intoa focused gas stream and imparted with enough velocity that the force ofimpact of particles with the substrate is sufficient to achieve adequatebonding.

[0006] Flame spraying and plasma spraying are conventional methods bywhich coatings are applied. Flame spraying involves the heating andprojecting of the coating material through the use of an oxygen fuelflame and pressurized carrier gas jet. Coating material is melted,atomized, or softened as it is fed into the flame, and the soft ormolten particulate is ejected in a directed stream through the nozzle ofthe spray gun toward the substrate. Like flame spraying, plasma sprayingalso heats and projects particles of a material toward a work surface orsubstrate A plasma gun replaces the flame used in older spray systemswith a stream of highly ionized inert gas, plasma, and imparts greatervelocity to the powder. In the plasma spray system, an electric arc iscreated and a mixture of combustible gases is ignited to create a hightemperature flame. The resulting plasma flame can be pushed forward infront of the gun. When powder is injected into the plasma flame near thefront of the plasma spray gun nozzle, the gases expand rapidly, and theresulting velocity of the heated powder particulate propels it to thesubstrate. In both flame spraying and plasma spraying, large sizeparticles or powders (typically >10 micrometers) are injected.

[0007] Coatings for protective and similar applications are formed byspraying hot powder particles onto a substrate to form coatings withthicknesses ranging from about ten up to hundreds of micrometers. Theresulting coating exhibits a relatively dense and smooth, orlow-profile, surface with large particle sizes. Thermally sprayedceramic coatings may exhibit physical characteristics such as a durable,higher profile surface suitable for gripping and anti-skid applications.In applications such as sliding wear situations, in which a smoothersurface is required, the coated surfaces may be further refined bygrinding and polishing.

[0008] Thermally sprayed coatings have a relatively weak bond with thesubstrate. Internal stresses, especially in thicker coatings, and highstress wear patterns may exceed the bond between the coating andsubstrate and cause failure. Surface treatments that are known in theart, such as grit blasting, enhance the work surface and provide ananchor profile for the coating so that it will better adhere to thesubstrate. The resulting high profile coating can be polished orotherwise refined as discussed above to achieve a smooth surface in, forexample, protective coating applications.

[0009] In the present invention, the thermal spraying process produceson a surface a small particle size and high surface area coating that isdesirable in applications where the coating is a catalyst itself, orwhere the treated surface is a precursor surface for a further coating.The thermal sprayed treated substrate surface requires no furtherrefinement.

[0010] A method for applying and forming a thermally sprayed coating ona substrate that yields a thin layer of a catalyst coating with smallparticle size and high surface area is the desirable object achieved bythe present invention. A coating applied to a substrate using thermalspraying techniques in which the coating has mechanical stability andpromotes catalytic reactions is yet another object achieved by thepresent invention

[0011] Catalytic material has been used as a coating on ceramic ormetallic substrates in applications such as, for example, catalyticconverters in automobile exhaust systems to reduce the emission ofnoxious gases. In the prior art, the substrate is typically coated witha catalyst by immersion in a slurry containing the catalytic material.Problems arise in that catalytic material does not adhere as well to ametallic substrate as it does to a ceramic substrate and the coating isnot uniformly distributed.

[0012] U.S. Pat. No. 5,721,188 to Sung et al. discloses a thermal spraymethod for adhering a catalytic material to a metal substrate. Thedisclosed method involves thermally spraying refractory oxide particlesonto a substrate for the primary purpose of attaining an undercoathaving high surface roughness, and the subsequent application of aseparate catalytic material to the undercoated substrate. Refractoryoxide powders ranging in average particle size from 13 to 180 micronsare thermally sprayed onto the substrate (See, for example, column 3,lines 27-32). Catalytic material is then applied to the undercoatedsubstrate by immersing the substrate in a wash coat slurry containingthe catalytic material. The coated substrate is then dried and calcined(Id., column 4, lines 30-35). Sung et al. recognized the need for amethod to improve the adhesion between metallic substrates and catalyticmaterials disposed thereon (Ibid., column 1, lines 29-31) but do notpropose a method for adhering a catalytic material directly to ametallic substrate.

SUMMARY OF THE INVENTION

[0013] The present invention provides a method of coating a substratesuch that the coating bonds directly with the surface of the substrate..Characteristics of the thermally sprayed coating, including highporosity, high surface profile and surface area, and small particlesize, are beneficially achieved in the application of the presentinvention. In the method of the present invention, a catalyst precursormaterial such as a powder having a large particle size, for example,greater than 10 micrometers, is thermally sprayed onto the substrate andforms a catalyst coating that bonds to the substrate surface. Thecoating is formed from decomposition products of the sprayed materialhaving small particle size, namely in the order of less thanapproximately 5 microns, and more specifically in the order of less thanapproximately 3 microns. The substrate may in turn be coated with afurther coating of a catalytic material. In a useful embodiment, thecoated substrate is used as a separator between alternate fluid flows ina micro-component reaction chamber, or heat exchanger, and the coatingacts as a catalyst to promote a chemical reaction in a fluid flowingover the substrate in the chamber

[0014] In one embodiment, the present method is applied to form acatalyst coating on a separator element in a micro-component heatexchanger such as that described in U.S. patent application Ser. No.09/627,267 filed on Jul. 28, 2000, entitled “Multi-purpose Micro-channelMicro-component,” the disclosure of which is hereby incorporated byreference. The catalyst material formed in the thermal spray promotes achemical reaction when a reagent fluid flows through the channels of themicro-component device. One or both sides of the separator may betreated. In this aspect of the invention, the coating, and the enhancedsurface properties thereof, also assist in the transfer of heat betweenfacing channels in the micro-component device.

[0015] In another embodiment, the method of the present inventioncomprises thermally spraying aluminum hydroxide particles onto a metalsubstrate. In yet another embodiment, the method may comprise thermallyspraying catalyst precursor material onto a surface, or oppositelyfacing surfaces of a metallic substrate. In the invention, a large sizepowder of a precursor material is flame sprayed or plasma sprayed onto asubstrate to produce a small particle size coating by the pyrolysisprocess, i.e., the decomposition by heat of the sprayed material.

[0016] Before treatment in accordance with the invention, the substratesurface may be enhanced by methods such as grit blasting to improveadhesion of the thermally sprayed coating to the substrate.

[0017] The invention is described more fully in the followingdescription of the preferred embodiment considered in view of thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows a representation of the thermal spray process (not toscale) in which a powder is introduced into a flame or plasma directedto a substrate surface.

[0019]FIG. 1A is a 400× magnified view of the surface of a metalsubstrate coated with alumina.

[0020]FIG. 1B is a 4000× magnified view of the surface of a metalsubstrate coated with alumina shown in FIG. 1A.

[0021]FIG. 2 represents a section of a steam reformer I heat exchangereaction chamber with a coating of the invention, as used in amicro-component assembly.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT

[0022] The present invention relates generally to a method for forming acatalyst coating directly on a metallic substrate. FIG. 1 shows arepresentation of the thermal spray process used in the invention. Apowder material 1 is introduced into a flame or plasma 2 that isdirected to the surface 3 of a substrate 4. A coating results whichprovides a porous, or enhanced surface area, on the substrate surface,which may in turn be coated with a further catalyst material such as anoble metal. Metal hydroxide, carbonate and nitrate materials are usedin the invention with thermal spray processes to coat metallicsubstrates and increase porosity and surface area. In a line of sightthermal spray, the materials decompose and oxidize by pyrolysis andproduce oxides that adhere to the substrate surface. The typicalreactions, Me(OH)_(x)→MeO_(x)+H₂O in the instance of metal hydroxides;Me(CO₃)_(x)→MeO_(x)+CO₂ in the instance of metal carbonates, andMe(NO₃)_(x)MeO_(x)+N₂O in the instance of metal nitrates, occur in thethermal spray process. The method treats a surface of a substrate bythermally spraying large size particles, >10 micrometers, of acomposition such as a metal hydroxide, carbonate, or nitrate directlyonto the substrate whereby a coating of small size particles, <5microns, and more particularly <3 microns, is formed on the substrate.The surface area and porosity properties of the substrate are enhanced.Substrates with metal oxide surfaces produced by the method are usefulas catalyst materials, particularly in micro-component assemblies.

[0023]FIG. 2 illustrates a section of a wavyplate or separator in amicro-component heat exchanger processed in accordance with theinvention. In the figure, a sinusoidal shaped waveplate 20 has twoopposite sides 21 and 22, respectively exposed to laminar fluid flows Aand B in a heat exchanger assembly such as described in U.S. patentapplication Ser. No. 09/627,267 referenced above. In an example, oneside 21 of the wavyplate is coated in accordance with the process hereinwith a catalyst material 23 to promote the steam reforming reaction influid flow A that may be generally characterized as: Hydrocarbon Fuel+H₂O→H₂+CO₂+H₂O+CO. Maintenance of the steam reforming reaction requiresthat heat be input into the exchanger. The sustained 700° C. heat forthe catalytic reaction is provided by an exothermic reaction in fluidflow B of a mixture of combustible materials, such as fuel cell off gasand/or gasoline in a mixture with air on the opposite side 21 of thewavyplate. Centerpoints of the sections (top to top) of the wavyplateare approximately 3.0 millimeters apart.

[0024] In a general description, the invention is a method for enhancingthe surface area and porosity properties of a substrate. Particles of amaterial capable of forming (by pyrolytic decomposition or thermalreaction) a metal oxide, such as hydroxide, carbonate or nitratecompounds of transition metals and rare earth metals, are thermallysprayed directly onto the surface of the substrate. The particlessprayed form a metal oxide and bond as a coating onto the substratesurface. The bonding mechanism includes mechanical bonding of thecoating to the substrate. The coating effected by the thermal spraytreatment is a catalyst itself, or may in turn be coated with a furthercatalytic material such as a noble metal.

[0025] The thermally sprayed material is selected from the groupcomprising metal hydroxides, metal carbonates, and metal nitrates, and,in an example is an Al(OH)₃ powder that decomposes to Al₂O₃ havingparticle sizes on the substrate surface in the nominal size distributionrange of approximately less than 1 micron to approximately 3 microns.The photomicrographs of FIGS. 1A and 1B illustrate a metal surfacecoated with Al₂O₃ particles deposited in accordance with the method andprocess of the invention. In FIG. 1B a metal surface is coated withAl₂O₃ particles in which 90% of the particles are less that 1 micron insize and 10% of the particles are in the size range between 1 micron and3 microns. It is understood that the range stated is approximate and mayvary, although the preferred object of the invention is a substratesurface coated with particles in a size, or sizes within the rangenoted. The surface to be coated may be treated before spraying, forexample, by grit blasting.

[0026] The invention finds use in the production of micro-componentassemblies used as heat exchangers or reaction chambers in variouschemical reaction processes. Coatings produced by the invention areadaptable to separator elements used in such assemblies where separatefluid flows pass on opposite sides of a separator. So Micro-componentassemblies thus treated are intended to be within the scope of thepresent invention.

[0027] The method is particularly useful, for example, as applied to aseparator in a micro-component heat exchanger having micro-channels forthe promotion of chemical reactions in fluid flows through the device,such as that disclosed and described in the previously referenced U.S.patent application Ser. No. 09/627,267, in which catalytic or steamreformer reaction chambers are provided on one or both sides of aseparator in a heat exchanger.

[0028] In examples, metal alloy substrates, such as a plate or foil shimwith thicknesses of approximately 60 micrometers, 100 micrometers, ormore are cleaned and prepared for surface treatment, for example, bywashing in trichloroethylene and ethanol and rinsing with deionizedwater. The surface may be grit blasted to increase roughness to a rangeof about 5 micrometers.

[0029] The coating is formed on the cleaned substrate by thermallyspraying powders that are catalyst precursors directly onto thesubstrate by thermal spraying processes known in the art. Flame sprayingand plasma spraying are species of thermal spraying. In a preferredembodiment, the catalyst precursor coating material is applied bythermal spraying a powder onto a substrate. During the spraying process,most Al(OH)₃ will decompose into small particles of Al₂O₃. Al(OH)₃ has alower melting temperature than Al₂O₃, hence, Al(OH)₃ requires a lowerflame temperature, a benefit in the treatment of a thin shim. The lowerthe substrate temperature, the more porous (greater surface area andsmaller particle size at the surface) the coating will be. In plasmaspraying, for example, a temperature in the order of 30,000 ° C. mayexist in the plasma, however, the peripheral regions of the spray wherepyrolysis of the introduced powder occurs involve reaction temperaturesin the range of approximately 2,000° C. to approximately 2400° C.Specific temperatures that will induce the pyrolytic decompositionreaction of the invention are dependent on particle size of theintroduced powder, its composition, the rate of introduction, and theporosity characteristics of the substrate (i.e., particle sizes on thesubstrate) ultimately desired after treatment.

[0030] A further consideration of using Al(OH)₃ is that the phasetransition after deposition of the coating on the surface during thecooling process is a transition from α to γ phase. This transitionprevents the formation of large crystalline clusters on the surface.Overall, by introducing Al(OH)₃ powder into the thermal spray, porousγ-Al₂O₃surface coatings are produced with particle size smaller thanapproximately 3 micrometers, as a result of the decomposition processduring coating, 2Al(OH)₃→Al₂O+3H₂O.

[0031] The flame temperature or plasma power required to achievesufficient mechanical bonding of the catalyst precursor material to thesubstrate varies depending on the specific catalyst precursor materialused. The appropriate temperature/power is material dependent, butshould be sufficient for pyrolysis of the sprayed material. Inalternative embodiments, a catalyst coating may be formed on thesubstrate by using any catalyst precursor that decomposes to a metaloxide (pyrolysis process), including but not limited to metalhydroxides, metal carbonates, and metal nitrates. The pyrolyticdecomposition of such materials may be represented by the formulas:

For a metal hydroxide: Me(OH)_(x)→MeO_(x)+H₂O

For a metal carbonate: Me(CO₃)_(x)→MeO_(x)+CO₂

For a nitrate: Me(NO₃)_(x)→MeO_(x)+N₂O

[0032] The pyrolysis of a metal hydroxide catalyst precursor material toa catalyst coating may be represented by the equation: 2Al(OH)₃→Al₂O₃ +3H ₂O. The catalyst precursor material is heated by the flame or plasmaof the thermal spray process and decomposes to a metal oxide en route tothe substrate forming a metal oxide catalyst coating.

[0033] The hydroxide, carbonate, or nitrate powders or particles usedprovide a large size precursor to produce a coating of small sizeparticles. While useful particle or powder sizes are not limited in theinvention, the normal size distributions for particles or powderssprayed in the invention are in the range of nominally greater than 10micrometers, and range from approximately 15 micrometers up toapproximately 200 micrometers. As illustrated in the following example,a coating of catalytic material applied to a substrate will exhibitsmall particle size and high surface area characteristics particularlyuseful with micro-channel micro-component assemblies.

EXAMPLE I

[0034] A substrate (a 5 centimeter by 10 centimeter shim) of a stainlesssteel alloy, Iconel® 625, with a thickness of approximately 100micrometers, was cleaned by washing with trichloroethylene forapproximately 30 minutes and ethanol for another approximately 30minutes and then soaked in deionized water for approximately 60 minutesin an ultrasonic cleaner.

[0035] The surface of the cleaned shim was enhanced by grit blastingusing alumina particles of approximately 220 mesh size to obtain aroughness of several micrometers.

[0036] The treated substrate was coated with alumina by sprayingaluminum hydroxide powder of nominal average particle size of about, butgenerally larger than, 15 micrometers in a plasma stream to provide acoating on the substrate surface having a thickness of about 10micrometers. The coating presented a γ −Al₂O₃ catalyst materialdetermined by XRD with low density and high porosity and surface areawith particle size on a micrometer to sub-micrometer scale in thedistribution. This process and materials of the Example provide asurface enhancement of 100 to 200 times as determined by BET.

[0037] Having described the invention in detail, those skilled in theart will appreciate that, given the present disclosure, modificationsmay be made to the invention without departing from the spirit of theinventive concept herein described. Rather, it is intended that thescope of the invention be determined by the appended claims.

I claim:
 1. A method for enhancing the surface area and porosity properties of the surface of a substrate comprising thermally spraying particles of a material capable of decomposing to a metal oxide directly onto the surface of the substrate.
 2. A method for enhancing the surface area and porosity properties of the surface of a substrate comprising plasma spraying particles of a material capable of decomposing to a metal oxide directly onto the surface of the substrate.
 3. A method for enhancing the surface area and porosity properties of the surface of a substrate comprising flame spraying particles of a material capable of decomposing to a metal oxide directly onto the surface of the substrate.
 4. A method for providing a coating of a catalyst precursor on the surface of a metallic substrate comprising plasma spraying particles of a material capable of decomposing by pyrolysis to a metal oxide directly onto the surface of the metallic substrate.
 5. A method for providing a coating of a catalyst precursor on the surface of a metallic substrate comprising flame spraying particles of a material capable of decomposing by pyrolysis to a metal oxide directly onto the surface of the metallic substrate.
 6. The method of claim 1 or claim 2 or claim 3 or claim 4 or claim 5 in which the substrate is coated with a noble metal catalyst after the spraying.
 7. The method of claim 1 or claim 2 or claim 3 or claim 4 or claim 5 wherein the material capable of decomposing to a metal oxide is selected from the group comprising metal hydroxides, metal carbonates, and metal nitrates.
 8. The method of claim 1 or claim 2 or claim 3 or claim 4 or claim 5 wherein the material capable of decomposing to a metal oxide is selected from the group comprising transition metal hydroxides, carbonates, and nitrates.
 9. The method of claim 1 or claim 2 or claim 3 or claim 4 or claim 5 wherein the material capable of decomposing to a metal oxide is selected from the group comprising rare earth metal hydroxides, carbonates, and nitrates.
 10. The method of claim 7 wherein the material is an Al(OH)₃ powder.
 11. A substrate coated with metal oxide particles having particle sizes in the nominal distribution range of less than approximately 5 microns.
 12. The substrate of claim 11 coated with metal oxide particles having particle sizes in the nominal distribution range of less than 1 micron to approximately 3 microns.
 13. A substrate coated with Al₂O₃ particles having particle sizes in the nominal distribution range of approximately less than approximately 5 microns.
 14. The substrate of claim 13 coated with Al₂O₃ particles having particle sizes in the nominal distribution range of approximately less than 1 micron to approximately 3 microns.
 15. The method of claim 1 or claim 2 or claim 3 or claim 4 or claim 5 further comprising treating the substrate surface to improve adhesion of the material sprayed onto the surface before the thermal spraying.
 16. The method of claim 15 wherein the treating comprises grit blasting the surface.
 17. The method of claim 1 or claim 2 or claim 3 or claim 4 or claim 5 wherein Al(OH)₃ is the material sprayed onto the substrate.
 18. The method of claim 17 wherein aluminum hydroxide decomposes into aluminum oxide and water according to the formula 2Al(OH)₃→Al₂O₃+2H₂O.
 19. A micro-component reaction chamber assembly including a separator with opposite surfaces intended to be exposed to separate fluid flows through the assembly in which at least one surface of the separator is treated in accordance with the method of claim 1 or claim 2 or claim 3 or claim 4 or claim
 5. 20. The assembly of claim 19 comprising a heat exchanger for separated fluid flows in which one side of the separator has a catalyst coating on one surface exposed to fluid flow.
 21. The assembly of claim 20 in which the surface of the one side includes a coating of Al₂O₃.
 22. The assembly of claim 21 in which the surface coating of Al₂O₃ includes particle sizes in the nominal distribution range of approximately less than 1 micron to approximately 3 microns.
 23. The micro-component reaction chamber assembly of claim 19 including a separator with opposite surfaces intended to be exposed to separate fluid flows through the assembly in which at least one surface is coated with metal oxide particles in the nominal distribution range of less than approximately 5 microns. 